The present invention relates to an apparatus for evaluating the characteristics of an optical fiber amplifier and more particularly to an apparatus for measuring the noise figure in optical amplifiers employed for large bandwidth applications such as dense wavelength division multiplexing systems.
Wavelength division multiplexing (WDM) optical communication systems employ optical amplifiers to increase the power of optical channels across a gain bandwidth. The gain (Pout/Pin) of an optical amplifier is measured by the ratio of the output power (Pout) of a signal to its input power (PIN). Amplifier gain efficiency measures the gain as a function of pump power in dB/mW. Obtaining accurate gain characteristics is subject, however, to amplifier noise. The dominant source of such noise is amplified spontaneous emission (ASE) which is produced by the spontaneous emission of photons in the active region of the amplifier. Because these amplifiers are used in long-distance transmission, such as submarine communication systems, they must have gain characteristics that are relatively flat and wide in addition to possessing low noise and high efficiency. This makes evaluation of the wavelength characteristics of an optical amplifier important to system design and implementation.
A typical testing configuration for an optical amplifier is illustrated in FIG. 1. An optical amplifier 20 is disposed between light source 10 and an optical spectrum analyzer (OSA) 30. Light from source 10 is amplified by amplifier 20 and measured by OSA 30. Because optical amplifiers suffer from ASE, noise is measured by the OSA before and after the amplifier. In this manner, a noise density is obtained for the source which is subtracted from the noise density measured after the amplifier to determine the noise associated with the amplifier (i.e. source subtraction). This method works well for low power, narrow-band amplifiers. As the power and bandwidth increases, the noise level from the input signal loading the amplifier likewise increases. However, a drawback associated with this testing configuration occurs when the input noise increases to the level of the noise generated by the amplifier. In this instance, it becomes difficult to separate out the noise generated by the amplifier from the noise associated with the input optical signal.
Accordingly, there is a need for an optical testing apparatus and method that overcomes the deficiencies of the prior art by measuring the noise figure in optical amplifiers employed for large bandwidth applications.
The present invention provides an apparatus for measuring the noise figure in optical amplifiers employed for large bandwidth applications such as dense wavelength division multiplexing systems. The testing apparatus provides a device under test with optical signals having the appropriate noise and power levels for the operation of such a device under test.
An optical amplifier evaluating apparatus consistent with the invention includes a light source that supplies a plurality of WDM optical channels across a predetermined bandwidth; a booster amplifier coupled to the light source is configured to increase the average power associated with each of the optical channels, the amplifier produces broadband noise across the predetermined bandwidth; a periodic filter is coupled to the booster amplifier where the filter has a periodic attenuation (e.g., reflection or absorption) characteristic configured to attenuate noise between each of the optical channels across the predetermined bandwidth; and a device under test coupled to the periodic filter.
In another aspect of the present invention, there is provided a method of testing an optical amplifier that includes the steps of outputting light signals corresponding to WDM optical channels from a light source; amplifying the average power of the light signals; filtering the amplified light signals such that noise between the optical channels is suppressed; supplying the filtered optical signals to a device under test; and measuring a noise figure associated with the device under test.